1. Field of the Invention
The present invention relates to a corneal endothelial cell photographing apparatus for photographing endothelial cells of a cornea.
2. Description of the Related Art
A corneal endothelial cell photographing apparatus disclosed in Japanese Patent Laid-open No. Hei 5-146410 projects a slit light beam obliquely on an eye to be tested, and receives the slit light beam reflected from the cornea by an image pickup device, such as a CCD camera to form an image of corneal endothelial cells.
This prior art corneal endothelial cell photographing apparatus has a photographing light projecting optical system that projects a photographing light beam obliquely on the eye to form an image of corneal endothelial cells of the eye. The photographing light projecting optical system is used also as an observation light projecting optical system for projecting an observation light beam obliquely on the eye to observe the corneal endothelial cells prior to photographing and a Z-alignment measuring light projecting optical system for projecting an alignment measuring light beam on the eye to measure the position of a main unit of the corneal endothelial cell photographing apparatus relative to the eye with respect to a Z-direction (forward-backward direction). The photographing light projecting optical system is provided with a slit plate to project a slit light beam.
The corneal endothelial cell photographing apparatus disclosed in Japanese Patent Laid-open No. Hei 5-146410 uses a single slit plate for projecting the photographing light beam, the observation light beam and the alignment measuring light beam. The photographing light beam, the observation light beam and the alignment measuring light beam passed through the slit of the slit plate are slit light beam of the same width. When the single slit plate is thus used for projecting the photographing light beam, the observation light beam and the alignment measuring light beam, it is impossible to achieve both photographing a wide area and an accurate alignment measurement.
If the width of the slit of the slit plate is reduced to increase the accuracy of the alignment measurement, the width of the photographing slit light beam is reduced accordingly and the narrow photographing slit light beam is able to illuminate only a limited photographing area. If the width of the slit of the slit plate is increased to illuminate an enlarged photographing area, the accuracy of the alignment measurement decreases.
A corneal endothelial cell photographing apparatus disclosed in Japanese Patent Laid-open No. Hei 8-117190 employs two slit plates, i.e., an observation slit plate provided with a narrow slit and a photographing slit plate provided with a wide slit, and uses the photographing slit plate in combination with a photographing light projecting optical system and the observation slit plate in combination with an observation light projecting optical system and a Z-alignment measuring optical system. Thus, a wide area can be photographed and Z-alignment measuring accuracy can be improved.
However, the corneal endothelial cell photographing apparatus disclosed in Japanese Patent Laid-open No. Hei 8-117190 has the disadvantage that only a narrow area of the corneal endothelium can be observed when observing corneal endothelial cells prior to photographing.
The corneal endothelial cell photographing apparatus disclosed in Japanese Patent Laid-open No. Hei 8-117190 uses the same slit plate by both the observation light projecting optical system and the Z-alignment measuring light projecting optical system, and the slit plate is provided with the narrow slit in view of achieving the accurate alignment measurement. Therefore, although an image of a wide area can be photographed as typically illustrated in FIG. 8, only a narrow area of a width smaller than that of an image formed by photographing can be observed. Indicated at S1 in FIG. 8 is a photographed image of corneal endothelial cells, and indicated at S2 in FIG. 9 is an observed image of corneal endothelial cells.